May lose data when not powered on frequently, because the storage of data is done in cells with electrons, and reading data means measuring the potential difference of the cell in volts. Leakage of electrons through the cell walls means that bits can flip from 1 to 0 or vice versa.
You should power on SSDs at least once a week to be safe, as data loss can already occur after 7 days1 of not powering the drive.
Total Bytes Written or TeraByte Written, the amount of data you can write to the SSD before it may fail. Usually the real amount of data which can be written is way higher than this metric (about 2 times), but SSDs may also fail way earlier.